Attention has been drawn to a hydrogen-oxygen fuel cell as a power generating system which presents substantially no adverse effects on the global environment because in principle, its reaction product is water only. Polymer electrolyte fuel cells were once mounted on spaceships in the Gemini project and the Biosatellite project, but their power densities at the time were low. Later, more efficient alkaline fuel cells were developed and have dominated the fuel cell applications in space including space shuttles in current use.
Meanwhile, with the recent technological progress, attention has been drawn to polymer fuel cells again for the following two reasons: (1) Highly ion-conductive membranes have been developed as polymer electrolytes and (2) it has been made possible to impart extremely high activity to the catalysts for use in gas diffusion electrodes by using carbon as the support and incorporating an ion exchange resin in the gas diffusion electrodes so as to be coated with the ion exchange resin.
However, a perfluorinated polymer having sulfonic groups to be used as a polymer contained in a membrane and an electrode usually has unstable functional groups such as —COOH groups, —CF═CF2 groups, —COF groups and —CF2H groups at some molecular chain terminals, and therefore, there was such a problem that a polymer gradually decomposes during long-term fuel cell operations, followed by decreasing the power generation voltage. In addition, there was such a problem that the fuel cell operation cannot be conducted because decrease of the mechanical strength due to the polymer decomposition, locally causes pinholes, breaking, abrasion or the like.
The above problems are caused by the presence of such unstable functional groups at some molecular chain terminals of a fluorine-containing polymer, and as methods for stabilizing such molecular chain terminals, for example, the following methods have been proposed.
A method of hydrothermal treatment of a tetrafluoroethylene/hexafluoropropylene copolymer (hereinafter referred to as a TFE/HFP copolymer) at a high temperature to convert —COOH groups to —CF2H groups (U.S. Pat. No. 3,085,083).
A method of decarboxination and fluorination of a fluorine-containing polyether having a low molecular weight by using fluorine gas in a liquid state or a state as dissolved in an inert solvent, to stabilize terminal groups (U.S. Pat. No. 3,242,218).
A method of shearing a TFE/HFP copolymer by a twin-screw extruder at a high temperature, followed by treating with fluorine gas (U.S. Pat. No. 4,626,587).
A method of treating a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (hereinafter referred to as a TFE/PFVE copolymer) by contacting it with fluorine gas in the form of pellets (JP-B-4-83).
A method of treating a TFE/PFVE copolymer by contacting it with fluorine gas in the form of granules (JP-B-7-30134).
A method of treating a TFE/HFP copolymer or a TFE/PFVE copolymer by contacting it with fluorine gas in the form of a pulverized product having an average particle diameter of from 5 to 500 μm (JP-B-7-5743).
A method of treating a TFE/PFVE copolymer by stirring a polymerization product obtained by solution polymerization or suspension polymerization in water, followed by contacting the resulting spherical granules having an average particle diameter of from 1 to 5 mm with fluorine gas (JP-A-10-87746).
A method of subjecting a TFE/HFP copolymer or a TFE/PFVE copolymer to reactive heat treatment with oxygen and water by a kneader (JP-A-2000-198813).
A method of carrying out treatment of a TFE/HFP copolymer or a TFE/PFVE copolymer by melt-kneading in the presence of oxygen and melt-kneading in the presence of water in a single kneader (JP-A-2002-249585).
However, such methods are not designed for treatment of a polymer having ion exchange groups or their precursor groups, but designed for stability of a fluorine-containing polymer at the time of heat forming. Here, in this specification, precursor groups for ion exchange groups mean groups convertible to ion exchange groups by e.g. hydrolysis, and precursor groups for sulfonic groups may, for example, be —SO2F groups or —SO2Cl groups.
As a method of improving the stability of a fluorine-containing polymer containing ion exchange groups or their precursor groups, a treating method has been proposed wherein a perfluoropolymer having sulfonic groups is put in a shaking tube coated with nickel or a stainless steel container and contacted with fluorine gas (JP-B-46-23245). However, by such a method, the treatment was not sufficient, and if a perfluoropolymer having sulfonic groups, treated by such a method, was used, there was a problem that although the voltage decrease in a fuel cell operation became small, it did not reach a level of at most 10 μV/h, and the sufficient durability could not be obtained.
Further, in such a treating method with fluorine gas, a peroxide test is described as an index for durability against polymer decomposition, wherein from 0.5 to 1.5 g of a polymer is immersed in 50 g of a fenton reagent solution containing 30% of an aqueous hydrogen peroxide solution and 10 ppm of bivalent iron ions at 85° C. for 20 hours, and the weight decrease is measured after drying. However, there was a problem that a polymer containing ion exchange groups is highly hygroscopic and cannot be measured with sufficient precision.